This invention relates generally to check valves employed to provide generally unidirectional flow through a fluid passageway. More particularly, the present invention relates to miniaturized check valves of a type which employs a spring biased ball to control the fluid flow through the valve.
The provision of a compact check valve suitable for use at both high and low flow rates entails satisfying a number of design constraints simultaneously. The present invention is a new and improved check valve or relief valve of compact form wherein the valve is directed to having a relatively low resistance to fluid flow. One of the principal obstacles to providing a low resistance to flow characteristic in a conventional check valve is the flow path through the valve mechanism which flow path tends to impinge the valve members and the spring return assembly to thereby add significantly to the resistance of the fluid flow. The low resistance to fluid flow of the present invention is in part a function of the improved fluid flow path through the check valve.
It is also generally desirable for a check valve to have a high gain characteristic wherein as the pressure is increased to open the check valve, for a unit of pressure increase, the valve opening area also increases by a corresponding incremental amount. For a check valve with an extremely high gain characteristic, any pressure increase in the free flow direction causes the valve to approach a maximum opening position with a very small restriction to flow in the free flow direction. However, implementing such a high gain characteristic in a conventional check valve tends to adversely effect the closing rate of the valve with favorable high gain and the valve closing rate being generally inversely related.
In the event that the pressure differential is reversed from the free flow direction, a mechanism is employed in a conventional check to return the valve to the closed position so that there will be no or minimal backflow through the valve. Naturally, the spring mechanism which conventionally functions to close the valve correspondingly diminishes the favorable gain characteristics of the valve. Consequently, the greater and thus more favorable the valve closure rate, the less favorable the gain characteristic of the valve. Conventional spring biased ball check valves implement a mechanical compromise wherein the return spring has a relatively low spring rate or pre-load whereby the spring biases the ball to the closed position with an acceptable closure rate and provides acceptable gain characteristics for the valve in the free flow direction.
The use of a spring having a relatively low spring rate introduces additional negative consequences in conventional check valves. A relatively low spring rate results in a valve mechanism which tends to be driven in oscillation as a result of the natural frequency of the mass of the spring and the valve and the mechanical spring rate of the valve system. At the same time, the hydrodynamic forces which are exerted through the valve also tend to be sinusoidal in nature and thus tend to excite and drive the valve in a natural frequency oscillation. Such valve oscillation produces a squealing or chattering condition which is deleterious to valve performance. The foregoing instability characteristics inherent in a low rate spring can be overcome by employing a valve spring having a higher spring rate which of course detracts from the high gain characteristics of the valve.
For check valves of highly miniaturized or compact configurations, it is also especially critical that fluids, at very low flow rates, flow through the valve for extended periods of time without contaminants accumulating on or around the valve seat. The foregoing contamination or silting condition prevents a valve from subsequently fully closing in the presence of a reverse differential pressure. If the reverse differential pressure is sufficiently high, the contaminants or silt can be effectively crushed or compressed. However, the required high reverse differential pressure ordinarily does not occur under conditions where the check valve is employed with normal reverse flow pressures. Naturally, if the spring pre-load is increased, the silting phenomena may be alleviated or minimized, but the opening pressure of the valve is adversely affected.
Accordingly, it is a principle aim of the present invention to provide a check valve of a highly compact configuration which valve has a relatively low resistance to fluid flow in the free flow direction, has structural features which alleviate adverse silting conditions, has a relatively stable spring system to alleviate valve chatter, has a favorable valve closure rate and has an improved free flow fluid path through the valve.